Characteristics Of Fill And Environment Evolution Since The Last Glacial Maximum In The Yangtze River Incised Valley Area

As a huge negative relief which be sensitive to continental-oceanic interaction,the fill include abundant information about geology and palaeoenvironment in the Yangtze River incised valley.So,by study the characteristics of incised valley fill,we can achieve the knowledge including sea-level fluctuations,interaction between terrestrial and coastal marine processes,and sedimentary environment evolution during geological epoch,and so on.Based on detailed study to ZKO1 and ZK02 boreholes,including lithology,sedimentary texture and structre,sediment grain-size parameter,foraminifera,TOC,AMS 14C dating and cone penetration test,we aim to summarize the sedimentary facies types and characteristics,discuss sedimentary environment evolution since the Last Glacial Maximum(LGM)in the Yangtze River incised valley area.On the whole,from Zhenjiang,Yangzhou area to the Yangtze River Estuary,the bottom boundary of the Yangtze River incised valley since the LGM be sloping to the south-east,but it has ups and downs in local area.Under the unconformity surface,the strata is formed in the Late Quaternary mid-age,on the contrary,it's the strata since the LGM.From bottom up it could be grouped into six sedimentary facies:fluvial channel,floodplain,estuary,shallow marine,delta,and tidal flat.Fluvial channel sediments be mainly consist of sand and gravel,show unstable sorting coefficient,saltation transport,lack of foraminifera and abundant charcoal.The flood plain sediments major consist of clay and slit with a few of shells and foraminifera,and sorting is poor and stable.Estuary sediments have variable grain-size characteristics in different areas,showing two types sediments with different size and rich in tidal structure.Meanwhile,it have abundant denthic foraminifera,but it abundance and types have great change.Shallow marine sediments mainly composed of muddy clay,have stable grain-size parameter distribution,suspension transport and most fine environmentally sensitive grain-size components.Muddy clay contain rich organic matter and slight odor.Denthic foraminifera most consist of temperate class types living in euryhaline nearshore,with stable abundance.Delta sediments which have a good sorting coefficient,mainly made up by silty fine sand and clay with saltation transport mode.Meanwhile its show complex structure,plentiful charcoal and shells.Besides,foraminifera content reach peak.Tidal flat mainly composed of silty clay and clay,have a small amount of plant roots and oxidation spot.The fill in the Yangtze River incised valley integrallty record environment evolution since the LGM.From bottom up,grain-size sequence show coarse grain,fine grain,and coarse grain,reflect the hydrodynamic condition variation including strong,weaker,stronger,weak and stronger.During the period,sea-level fluctuations presented as periodic change,including low sea-level,transgression,high sea-level,regression.With the corresponding,sedimentary environment have a transitional evolution including continental facies,transitional facies,marine facies,transitional facies.Besides,from the land to the sea,fluvial channel facies and shallow marine facies gradually thickening.Cone penetration test can effectively reflect vertical variations of fill in the Yangtze River incised valley since the LGM.Different lithology have different cone penetration test characteristics including numerical indices,curve patterns and curve positional relationship.The cone penetration test characteristics of same lithology have similarity and some regularity.